The only true way is to have the analog signal from the Plasma be isolated (analog isolation) . Most commercial PLC's don;t offer that. Vaporizing metal at 30,000 degs with a high current arc creates LOTS of noise. Even WITH good grounding it can be a challenge to read the actual DC voltage and block the noise. Then you have the HF start and High voltage start systems that will burn up downsteam analog systems.
The post is correct in most aspects. Sometimes the best ground is no ground at all. Keeping galvanic separation keeps marginal grounding or potential ground loops to a minimum. even engineers argue among them selves about which is better: One common ground or isolated circuits with separate grounds. It is indeed all about inductance and impedence. Even short wires under heavy current become transmitters of noise.
TOM CAUDLE
www.CandCNC.com